The past fifteen years have seen an explosive growth of research interest in the study and application of organic materials as the active media in organic opto-electronic devices. Today, this work has culminated with organic light emitting devices (OLEDs), and specifically phosphorescent OLEDs. Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate.
Lasing action has been demonstrated in an organic laser having optically-pumped slab waveguide structures of vacuum-deposited thin films of small molecular weight organic semiconductors. V. G. Kozlov et al., Conf: on Lasers and Electro-optics CLEO '97, CPD-18, Opt. Soc. Am., Baltimore, Md., May 1997. Lasing from molecular organic as well as polymeric thin films has been shown to span the visible spectrum, extending into the near IR simply by making modifications to the lasing medium [V. G. Kozlov, V. Bulovic, P. E. Burrows, V. Khalfin, and S. R. Forrest, “Unique optical properties of organic lasers,” presented at CLEO '98, San Francisco, 1998]. More importantly, the characteristics of organic lasers are remarkably temperature independent [V. G. Kozlov, V. Bulovic, and S. R. Forrest, “Temperature Independent Performance of Organic Semiconductor Lasers,” Appl. Phys. Lett., vol. 71, pp. 2575, 1997]. For example, the temperature dependence of a DCM2 doped Alq3 optically pumped organic thin film laser is compared to a GaAs-based laser. The threshold for optically pumped lasing, as well as the lasing wavelength of a molecular organic thin film as a function of temperature show minimal temperature dependence, in contrast to a conventional GaAs-based laser. The almost complete lack of change in these parameters (including slope efficiency) is due to the isolated, quantum nature of the excited state in organic thin films. These molecular states are largely isolated from those of the environment, leading to lack of sharing of electrons in broad energy bands as occurs in inorganic semiconductors.
It is becoming increasingly apparent that the conventional techniques applied to achieving electrically induced laser emission in inorganic semiconductors, such as the use of an intensely pumped double heterostructure, may not be suited for the successfully generation of the laser emission in organic semiconductors. In organic materials, losses related to thin film resistance, polaron quenching and absorption, and singlet exciton annihilation may ultimately prohibit an organic thin film from reaching the lasing threshold by conventional approaches. [M. A. Baldo, R. J. Holmes, and S. R. Forrest, “Prospects for electrically pumped organic lasers,” Phys. Rev. B, vol. 66, pp. 035321, 2002; V. G. Kozlov, G. Parthasarathy, P. E. Burrows, V. B. Khalfin, J. Wang, S. Y. Chou, and S. R. Forrest, “Structures for Organic Diode Lasers and Optical Properties of Organic Semiconductors Under Intense Optical and Electrical Excitations,” IEEE J. Quant. Electron., vol. 36, pp. 18, 2000].
Perhaps the most promising and least explored option involves exploiting cavity polariton formation in organic materials embedded in high Q dielectric microcavities for the generation of coherent radiation. The study of microcavity polaritons in conventional inorganic semiconductors has been intensive since their initial discovery in GaAs in 1992 [C. Weisbuch, M. Nishioka, A. Ishikawa, and Y. Arakawa, “Observation of the coupled exciton-photon mode splitting in a semiconductor quantum microcavity,” Physical Review Letters, vol. 69, pp. 3314, 1992]. Recently, microcavity polaritons in organic materials have been described [N. Takada, T. Kamata, and D. D. C. Bradley, “Polariton emission from polysilane-based organic microcavities,” Applied Physics Letters, vol. 82, pp. 1812, 2003; P. Schouwink, H. V. Berlepsch, L. Dahne, and R. F. Mahrt, “Observation of strong exciton-photon coupling in an organic microcavity,” Chemical Physics Letters, vol. 344, pp. 352, 2001; D. G. Lidzey, D. D. C. Bradley, M. S. Skolnick, T. Virgili, S. Walker, and D. M. Whittaker, “Strong exciton-photon coupling in an organic semiconductor microcavity,” Nature (London), vol. 395, pp. 53, 1998].